This invention relates to a continuously variable traction drive speed changer utilizing a rolling wheel.
One example of a conventional traction drive rotary motion transmission mechanism for a continuous variable speed changer is the troidal-type. The structure of one typical example will be explained in conjunction with FIG. 1, which is a sectional view illustrating the continuously variable speed changer of this kind. In FIG. 1, 1 indicates a main drive shaft rotatably supported by the main frame 2 and connected to an external main drive unit (not shown). 3 is a main rolling wheel secured on the main drive shaft 1 for rotation therewith, the main rolling wheel 3 having formed therein a rolling surface 3a defined by a continuous circumferential groove of an arcuated cross section. 4 is an intermediate rolling wheel for transmitting the rotational drive force of the main rolling wheel 3 to a follower rolling wheel which will be described later. The intermediate rolling wheel 4 has formed therein a rolling surface 4a defined by a continuous circumferential ridge of an arcuated cross section and rotatably supported by an intermediate rolling wheel pivot shaft 5. 6 is a follower shaft connected to an external follower unit (not shown), and the follower shaft 6 is rotatably held with respect to the main frame 2. 7 is a follower rolling wheel for transmitting the rotational force on the main rolling wheel 3 to the follower shaft 6, the follower rolling wheel 7 having formed therein a rolling surface 7a defined by a continuous circumferential groove of an arcuated cross section. 8a is a contact pressure means for bringing the rolling surface 3a and the rolling surface 4a into contact under a predetermined contact pressure on a line S of action of the contact pressure, and 8b is a contact pressure means for bringing the rolling surface 7a and the rolling surface 4a into contact under a predetermined pressure on a line SS of action of the contact pressure. 9 is a speed changing means for the rotary rocking motion of the intermediate rolling wheel pivot shaft 5 about a point P to tilt the intermediate rolling wheel 4, so that rolling surface contacting regions on the contact pressure acting lines S and SS can be shifted. This speed changing means 9 is connected to the intermediate rolling wheel pivot shaft 5 through the gear 9a.
The operation of the conventional traction drive speed changer will now be described. When the unillustrated external main drive unit rotates the main drive shaft 1 and the main rolling wheel 3, the main rolling wheel 3 drives the intermediate rolling wheel 4 due to the friction between the rolling surfaces 3a and 4a. The rotation of the intermediate rolling wheel 4 causes the follower rolling wheel 7 to be driven due to the friction between the rolling surfaces 4a and 7a. Thus, the rotational force on the main drive shaft 1 is transmitted to the driven shaft 6. The speed changing operation of the conventional speed changer as above described is achieved by operating a speed changing means 9 including a handle 9b. That is, when the handle 9b is actuated to shift, through a sector gear 9a, the position of the points of contact between the rolling surface 3a of the main rolling wheel 3 and the rolling surface 4a of the intermediate rolling wheel 4 as well as the rolling surface 4a of the intermediate rolling wheel 4 and the rolling surface 7a of the follower rolling wheel 7, the effective diameter of the main rolling wheel 3 and the effective diameter of the follower rolling wheel 7 are changed, thereby achieving the speed changing.
The continuously variable traction speed changers in general including the above described conventional arrangement are operated with the following two fundamental requirements. The first of the requirements is that a contact pressure should be exerted at the rolling surface contact area by any contacting pressure generating means so that a friction is provided between the rolling wheels in pair, and the second requirement is that an operational force is applied to the rolling surface contact area by any speed changing means so that the contact area is shifted. The contact pressure should have a magnitude that provides the necessary frictional force necessary for transmitting torque at each time point and that is sufficient. An excess amount of contact pressure is rather disadvantageous in that it provides friction losses which are useless in power transmission. Therefore, it is necessary to decrease such excess contact pressure in order to ideally increase the transmission efficiency of the apparatus. Also, the above operational force is important in determining the control function of the apparatus, and if this operational force can be varied by any suitable speed changing means in relation to the transmission torque at each time point, an ideal continuously variable speed changing function results in which the transmission speed changing ratio is automatically controlled in accordance with the transmission torque.
However, if the ideal function as just discussed above is to be obtained with the conventional technique, it is necessary to further additionally provide special measures to each of the contact pressure generating mechanism and the speed changing mechanism.